The purpose of this project is to examine bone marrow grafts between adult mice with genetically defined antigenic differences to determine why some grafts succeed and to find which assays of donor-host compatibility best predict success of the marrow grafts. Erythrocyte producing tissues of genetically anemic mouse recipients are completely populated by grafts of marrow cells from normal donors, in many cases even when the donors have been specially bred to differ from the recipients at particular antigen-determining loci. No immune suppressive treatments are necessary although both marrow donors an recipients are immune-competent adults. Successful marrow grafts permanently cure the recipient's anemia. Their growth can be quantitatively determined by following changes in blood values to normal levels. Genetically defined anemic recipients will be given marrow grafts from donor mice each carrying one of the large numbers of different antigen-determining loci available on otherwise identical mouse stocks at the Jackson Laboratory. Anemic mice cured by incompatible donors will be carefully studied to understand what happens when the incompatible marrow graft succeeds. The various donor-recipient combinations in which marrow grafts suceed and those in which grafts fail will be tested by a battery of compatibility assays to determine which assays best measure the histocompatibilities necessary for marrow graft success. Successful marrow grafts do not seem to react against the recipients although the recipients have a number of alleles at antigen-determining loci which the donors lack. The absence of graft vs. host reactions will be analyzed and other donor recipient combinations will be tested.